1. Field of the Invention
Typically, a double overhead camshaft engine (referred to hereinafter as a DOHC engine) is provided with a pair of overhead camshafts for a row of cylinders. Such a pair of overhead camshafts usually includes an intake camshaft and an exhaust camshaft, which are arranged parallel to a crankshaft of the engine. One of the overhead camshafts, called a drive camshaft, is connected or coupled to the crankshaft by a belt which transmits the engine output to drive the drive camshaft The other of the overhead camshafts, called driven camshaft, is connected or coupled to the drive camshaft by transmission means, such as meshing camshaft gears secured to the drive and driven camshafts, respectively The transmission means transmits the rotation of the drive camshaft to drive the driven camshaft. To operate, or drive, valves with the overhead camshaft, a valve drive mechanism or valve train, including cams having cam lobes, is provided. Each cam lobe drives one valve.
2. Description of Related Art
A valve train such as that referred to above is known from, for instance, Japanese Unexamined Utility Model Publication No. 61- 171,807. Each valve drive mechanism of this publication cooperates with a hydraulic valve lash adjuster, which supports and urges a rocker arm disposed between a cam lobe of the overhead camshaft and a valve stem so as to maintain zero valve stem to rocker clearance. Such a hydraulic valve lash adjuster is described in, for example, Japanese Unexamined Utility Model Publication No. 55- 144803.
The drive overhead camshafts of the DOHC engine are supported for rotation by supporting means provided on a cylinder head. Camshaft supporting means of this kind typically comprise two bearing parts for supporting, for rotation, the camshafts therebetween. Such supporting means may include cam carrier means, provided separately from the cylinder head and bolted, or otherwise secured, to the cylinder head, and cap means, formed integrally with a cylinder head cover.
Since a camshaft drive mechanism as described above narrows a space between the drive and driven camshafts, a DOHC engine of this kind can be provided with a reduced width. On the other hand, the camshaft supporting means can be formed from a reduced number of parts and, consequently, allow the DOHC engine to be simple in structure.
In recent years, DOHC engines have typically been provided with a plurality of intake valves and a plurality of exhaust valves for each cylinder in order to increase intake charging efficiency and develop an increase in output power. Some DOHC engines of this kind have a different number of intake and exhaust valves for each cylinder.
The provision of a plurality of intake valves and a plurality of exhaust valves for each cylinder, and of an individual hydraulic valve lash adjuster for each valve, somewhat conflicts with a fundamental demand in car design for DOHC engines which are small in size. In particular, the cylinder head of a small DOHC engine must be formed with a plurality of bores and holes for installing the valves and valve trains, including the hydraulic valve lash adjusters, which unavoidably causes a decrease in structural rigidity of the DOHC engine body.
Oil, which lubricates the camshafts and valves and operates the hydraulic valve lash adjusters, scatters over a cylinder head during engine operation and produces oil mist. With an increase in the number of intake and exhaust valves and hydraulic valve lash adjusters, the quantity of oil mist on the cylinder head increases. Accordingly, blow-by gas, which is introduced into an oil separator, contains an increased quantity of oil mist, so that it is necessary to provide an oil separator of large capacity in order to process the blow-by gas efficiently. A large capacity oil separator necessarily occupies a large space, even though the DOHC engine is designed to be small in size.
In-mesh camshaft gears, for operationally coupling the drive and driven camshafts, are covered by a gear cover so as to prevent both foreign articles from being caught between the camshaft gears and lubrication oil from being scattered from the camshaft gears. In addition, the gear cover, if it is secured to the DOHC engine with the cam carrier means, is typically rigidly connected to the cam carrier means. For easy connection between the gear cover and cam carrier means, the gear cover is usually constructed as two parts which are separable in a direction parallel to the axis of the crankshaft. That is, the gear cover comprises a front cover section and a rear cover section formed integrally with the cam carrier means and bolted at several points around the peripheries of the camshaft gears or otherwise secured to each other, so as to enclose marginal portions of the camshaft gears.
To improve the rigidity of the camshaft supporting means and, in particular, parts of the camshafts near the camshaft gears, the cam carrier means is formed by radial bearing means and thrust bearing means. Since the camshaft gear has a diameter larger than diameters of the related camshaft and cam lobes, the camshaft gear projects downward on a side of the cylinder head. In order to eliminate an interference between the camshaft gear and an upper end of the cylinder head, the cylinder head is formed in an upper end portion with a recess for receiving lower parts of the in-mesh camshaft gears. That is, the in-mesh camshaft gears are accommodated in a space defined between the gear cover and the end recess.
Since it opens downward, the gear cover, or cover member, is low in rigidity. In addition, since the cover member is integral with the camshaft supporting means, it receives external loads from the camshafts, and is apt to cause a large, three dimensional deformation, owing to a change of torque of the camshafts, abnormal operations of the valve means, such as jumping and bouncing, or changes in angles of torsion of the camshafts, for example.